LANSING, Mich. (AP) — The country's top environmental regulator warned the Michigan governor and the Flint mayor of "systemic" problems that threaten the long-term ability to provide safe drinking water in the city after its lead contamination crisis subsides.

In a letter Thursday, Environmental Protection Agency Administrator Gina McCarthy said Flint's distribution system is too big for demand, letting water stagnate in pipes and potentially preventing chlorine from fighting disease-causing pathogens. She also told Gov. Rick Snyder and Mayor Karen Weaver that the water treatment plant is understaffed and lacks enough experienced employees.

Her other concerns included whether Flint administrators would be able to reliably and quickly provide administrative support. She also noted the need for a "solid and realistic" financial plan in a city whose residents pay some of the highest water bills in the U.S.

"The principal issues that must be addressed for long-term system reliability are systemic issues that go well beyond what is necessary to deal with the immediate crisis," McCarthy wrote.

She said federal, state and city officials have made strides to address the public health emergency by reducing lead levels and improving other key water-quality parameters. But she cited "significant challenges" in the long term that "will require the joint, sustained effort of both the city and the state."

The city of nearly 100,000 people has been dealing with lead contamination since switching from Detroit's water system, which draws from Lake Huron, to the Flint River in April 2014. The short-term move was made to save money during the construction of a Flint-area pipeline to the lake, which is still being built. But state regulators failed to require corrosion chemicals, which let toxic lead from old pipes leach into the supply.

In January, the EPA issued an emergency order demanding that the state of Michigan take a series of steps to make the water in Flint safe.

Flint has returned, for now, to its earlier source of treated water. The EPA said in its letter that if the city switches again it must show that it has the technical, managerial and financial capacity to run the system in compliance with the U.S. drinking water law.

Snyder spokeswoman Anna Heaton said Friday that the letter and an accompanying report were being reviewed.

"The ultimate decision for a new water source and management of that source rests with the city," Heaton said. "To support the city until it reaches that point, the state has provided the funding for the city to remain on Detroit's water system through the end of 2016, as well as corrosion control specialists and lab testing personnel to ensure the safety of the water supply."

A message seeking comment from a Flint spokeswoman about the letter wasn't immediately returned.